As is generally known, on the one hand, the higher the compression ratio of an internal combustion engine, the better the fuel consumption rate can be reduced. On the other hand, during high engine operation, there is an increased tendency for engine knock to occur. Therefore, on engines equipped with variable compression ratio mechanisms, in order to ensure a reduced fuel consumption rate, while avoiding undesirable knocking from occurring, the compression ratio must be controlled in such a manner as to be increased to a high compression ratio during low engine-load operation, and to be reduced down to a low compression ratio during high engine-load operation. One such variable compression ratio mechanism equipped engine has been disclosed in Japanese Patent Provisional Publication No. 7-229431 (hereinafter is referred to as “JP7-229431”). The variable compression ratio mechanism or the compression ratio controller disclosed in JP7-229431, includes a variable volumetric-capacity chamber, which is opened to the combustion chamber, and whose volume can be variably adjusted by means of a slidable compression-ratio control piston that is movable in a direction the volume of the variable volumetric-capacity chamber increases during the high load operation and in a direction the volume of the variable volumetric-capacity chamber decreases during the low load operation.
Instead of using the variable volumetric-capacity chamber attached onto the combustion chamber, the compression ratio can be adjusted by varying a piston stroke characteristic, that is, a top dead center (TDC) and/or a bottom dead center. A variable compression ratio mechanism of a reciprocating internal combustion engine, capable of variably controlling the compression ratio by varying the piston stroke characteristic, has been disclosed in Japanese Patent Provisional Publication No. 2002-21592 (hereinafter is referred to as “JP2002-021592”).
Generally, when changing the engine/vehicle operating condition from the steady-state running state to the accelerating state with an accelerator opening increasing in a stepwise manner, a quantity of air drawn into engine cylinders tends to increase with a response delay such as 0.1 seconds as compared to a timing of the driver's accelerator depression. On the other hand, in the systems as disclosed in JP7-229431 and JP2002-021592, in such high-load transient maneuvers, the compression ratio is reduced for the purpose of knocking avoidance, but the response in compression-ratio change occurring based on the compression ratio control is generally slower than that in intake-air quantity change occurring based on the intake-air quantity control. In case of the relatively slower response in compression-ratio change, although the quantity of fresh air drawn into engine cylinders reaches a stationary level, the systems as disclosed in JP7-229431 and JP2002-021592 exhibit a transient compression-ratio response that the compression ratio does not completely drop down to a designated low compression ratio. This means a possibility of undesirable engine knocking, occurring during such high-load transient maneuvers. The avoidance of engine knocking, occurring due to the relatively slower response in compression-ratio change, would be desirable even in transient maneuvers such as during a vehicle's accelerating condition.